IN PASSING THROUGH THE ATMOSPHERE. 
229 
$ 
11. The definition of transparency, according to Bouguer, is the reciprocal of the 
thickness of the medium, required to diminish the incident light in a given ratio. 
12. In the same year with the posthumous publication of Bouguer, appeared the 
“ Photometria” of Lambert; a work of great ingenuity and labour, based in many 
respects upon Bouguer’s earliest investigations already mentioned, to which the 
author fails not to give due credit. This work, owing to some circumstance, 
has always been very scarce : Priestley appears never to have been able to get 
a copy, and now it is one of the rarest of modern scientific works. It was printed 
under the title of “ Photometria, sive de Mensura et Gradibus Luminis, Colorum, et 
Umbrae. Augustae Vindelicorum, 1760, 8vo, pp. 547”*. In the first chapter of the 
fifth part, the author treats of the transparency of the atmosphere. His methods of 
finding the thickness of air traversed at different altitudes are less accurate approxi- 
mations than that of Bouguer. His method of observing the intensity of radiation 
was very different, as well as the result. Pie observed the degree at which a ther- 
mometer lying in the sun rose above one in the shade, and he took this difference 
as a measure of solar radiation. The experiments briefly cited in the “ Photometria ”~j~ 
are fully detailed in his “ Pyrometria,” published in quarto in 1779^. Lambert’s obser- 
vations on the intensity of the solar rays were made during a considerable part of the 
year. The following made at Coire on the 17th of May 1756, he selects as the best : 
the height of the barometer was twenty-six French inches. 
Sun’s Altitude. 
Reaum. 
Excess in the Sun. 
60 15-8 
50 14-6 
40 12-8 
30 10 0. 
Hence, by a procedure the same as has been above explained, he deduces the light 
transmitted through the atmosphere in a vertical direction, which he estimates at 
3 4 
0 5889 of the incident rays, being less than y instead of more than y, as Bouguer 
had supposed. 
13. Lambert’s work on Pyrometry is further remarkable for the clear description 
of several methods and results which have in later times been rediscovered or suc- 
cessfully applied. He was the first to apply calculation to the flow of heat through 
solid bodies § : he discovered the law of the intensity of radiant heat proportional to 
the sine of the angle made with the heated surface ||. He was aware that the true 
measure of the cause of radiant heat was the velocity of heating resulting from it^[. 
He applied this method to the determination of the permeability of successive plates 
of glass to the solar rays, constructing for each experiment the curve of rise of tern- 
* The analysis of this work in Montucla is imperfect, 
t page 158. § 283. § Pyr. § 327. 
|] Pyr. § 319. 
t page 396. § SS6. 
<[ § 270. 
